The present invention relates to large dynamoelectric machines and, more particularly, to the stators of the large electric generators which are commonly the armatures of the generators.
As is well known, large electric generators employ a stator built up by stacking thin laminations of magnetic material to form a generally annular assembly. An array of axial slots are formed in the radially inner surface of the annular assembly. Each axial slot contains at least one and preferably two armature bars which form part of the armature winding in which the electric output is induced.
A rotor is arranged to rotate coaxially within the stator. The rotor conventionally contains field windings which are excited by a DC excitation source of relatively low voltage (from about 300 to about 700 volts) in order to produce a magnetic field rotating at the same speed as a rotor. Prior art excitation sources include shaft-mounted DC generators, shaft-mounted AC alternators feeding rectifiers and external sources of excitation power. The excitation power from external sources is fed to the rotating field windings through slip rings. One popular type of external sources makes use of the AC output of the generator itself which, after passing through a transformer to provide the desired excitation voltage, is rectified and fed through slot rings to the field windings.
A more recent excitation system employs potential windings in, for example, three stator slots spaced 120 degrees apart. The three potential windings, which are hereinafter referred to as "P" bars, are exposed to alternations of the magnetic field which induces an AC voltage therein. The three "P" bars, being spaced 120 mechanical degrees apart therefore comprise a three-phase source whose output can be transformed as necessary to establish a desired excitation voltage which may then be rectified and the resulting DC fed through slip rings to the field windings. A self-excited system of this type is described in a paper by Cotzas et al entitled "GENERREX-PPS (Potential Power Source). Excitation System for Wisconsin Power & Light, Edgewater 5", presented at the Forty-Third Annual American Power Conference, Chicago Ill., Apr. 27-29, 1981. The GENERREX-PPS system provides a compact equipment giving reduced space requirements and simplified maintainability of a large generator. Control of the generator field voltage is accomplished by high-speed thyristor action. Further details of the GENERREX system do not concern the present disclosure and are therefore omitted.
The stator armature bars are conventionally clamped in the stator slots using dovetail grooves near the radially inner ends of the slots into which wedges are firmly engaged. The wedges impose radial forces on the armature bars for resisting radial forces on the armature bars due to normal and abnormal magnetic and electrical forces imposed on them.
The "P" bars are conventionally disposed radially inward of the wedges between the wedges and the surface of the rotor and are tied in place using, for example, a fiber roving pre-impregnated with a heat-curable resin which is cured after the roving is installed. One such method for securing "P" bars in place is disclosed in U.S. Pat. No. 4,385,252.
Securing the "P" bars atop the wedges imposes certain penalties. The need to manually wrap ties every few inches about the "P" bars and the wedge and to then cure the resin in the ties adds substantially to the labor required for assembly of the generator. In addition, after a generator has been in service for a substantial period, the stator may require refurbishing. Since the three "P" bars are atop the wedges, replacement of the wedges in the three slots contain the "P" bars requires cutting the roving ties and removing the "P" bars before removing and replacing the wedges. In order to remove the "P" bars, their coolant piping and end turns must be disconnected. Since the "P" bars are on the order of an inch or two in cross section and as much as 25 or more feet long, the chance of damaging them during removal is quite high. After new wedges are in place, the "P" bars must again be installed, tied in place and cured. These penalties all equate to adverse labor cost and/or schedule impact.
One technique for securing "P" bars in stator slots is disclosed in U.S. patent application Serial No. 656,107 filed on the same date as the present application. In this disclosure, each stator slot includes an inner pair of the dovetail slots and an outer pair of dovetail slots. In all stator slots except those containing "P" bars, the wedges are installed in the inner pair of dovetail slots to secure the armature bars in place. In stator slots containing the "P" bars, the "P" bars are installed on top of the armature bars inside the slots and the wedges are installed atop the "P" bars in the outer pair of dovetail slots.
In order to reduce heating of the ends of the stator core due to stray axial magnetic fields, it is desriable to outwardly taper the last several inches of the stator core. The outward taper intersects the outer pair of dovetail slots thus reducing or eliminating support of the "P" bars and underlying armature bars in the outer several inches of the stator slots containing the "P" bars. The outward taper may not intersect the inner pair of dovetail slots and thus the armature bars contained in the stator slots not containing "P" bars may not be satisfactorily supported to the axial ends of the stator.